Vectors and For Loops

Much of the time, you will be using for loops to manipulate vectors. Once you are comfortable using for loops with vectors, you can do things like:

• populate a vector with values
• do math with vectors

Here is a program that initializes a vector and then uses a for loop to populate the vector with values. Then another for loop reads out the vector values.

#include <iostream>
#include <vector>

using namespace std;

int main() {

    vector<float> example;

    for (int i = 0; i < 5; i++) {
        example.push_back(i*5.231);
    }

    for (int i = 0; i < example.size(); i++) {
        cout << example[i] << endl;
    }

    return 0;
}

The output looks like this:

0
5.231
10.462
15.693
20.924

Using i++ Versus ++i

So far, you've learned to write C++ for loops like the following:

for (int i = 0; i < 10; i++) {}

This syntax matches closely to the Python for loop syntax; however, you can also write a for loop like this:

for (int i = 0; i < 10; ++i) {}

What is the difference and why do both ways work?

In practice, both i++ and ++i will give you the same results; these are a shorthand way of writing i = i + 1. The difference between the two is subtle.

int i = 5;
int x = i++; // x = 5, i = 6 (called postfix)
int x = ++i; // x = 6, i = 6 (called prefix)

In both cases, the i variable increases by 1. In the postfix case, i++, int x = i is evaluated first and then i = i + 1 occurs.

In the prefix case, ++i, i = i + 1 occurs first and then int x = i executes.

Many code guidelines recommend using ++i over i++. In reality neither one is more efficient than the other when using integer variables.

However, there is a difference when you write a C++ class that overloads the ++ operator. You saw operational overloading in the Python matrix project; the code overloaded mathematical signs to carry out matrix addition, subtraction, multiplication, etc.

When overloading the postfix operator, C++ needs to keep track of two values. In the example, the values would be 5 and 6. For the prefix operator, C++ only needs to keep track of one value: 6. Hence, when overloading the ++ operator, it's generally more efficient to use prefix than the postfix.

Overloading is an advanced C++ topic that isn't covered in depth here. If you'd like to learn more, here are a few resources:

Stackoverflow

IBM Knowledge Center

Practice with Vector Methods and For Loops

Start Quiz:

//TODO: include the iostream and vector libraries

//TODO: Use the standard namespace

int main() {
    
    // Part 1: declare and define a vector with values
    //        {5.0, 5.0, 5.0} and print 
    //         the vector to the terminal using cout
    // Hint: the syntax vector<datatype> varname(length, value) might help

    // Part 2: Use push back to add the values 3.0, 2.5, 1.4 
    //      to the back of the vector

    // Part 3: Print out the vector again using cout

    // Part 4: Use the vector assign method to override the current vector. 
    // The overriden vector should have 3 elements with 
    // the values {5.0, 5.0, 5.0}

    // Part 5: Print out the vector

    return 0;
}

#include <iostream>
#include <vector>

using namespace std;

int main() {
    
    // Part 1: declare and define a vector {5.0, 5.0, 5.0} and print it out
    vector<float> vectorvar(3, 5.0);
    
    for (int i = 0; i < vectorvar.size(); i++) {
        cout << vectorvar[i] << " ";
    }
    
    cout << endl;
    
    // Part 2: Use push back to add the values 3.0, 2.5, 1.4 to the back of the vector
    vectorvar.push_back(3.0);
    vectorvar.push_back(2.5);
    vectorvar.push_back(1.4);
    
    // Part 3: Print out the vector
    for (int i = 0; i < vectorvar.size(); i++) {
        cout << vectorvar[i] << " ";
    }
    cout << "\n";
    
    // Part 4: Use the assign method so that the current vector has values 
    // {5.0, 5.0, 5.0}
    vectorvar.assign(3, 5.0);
    
    // Part 5: Print out the vector
    for (int i = 0; i < vectorvar.size(); i++) {
        cout << vectorvar[i] << " ";
    }
    
    cout << "\n";

    return 0;
}